text:
Would a stationary WWII fighter fall on its nose with full throttle and brakes on?
Would a stationary WWII fighter fall on its nose with full throttle and brakes on?
It's a general question, but I am interested in the Republic P-47.

When you say "full throttle", I assume you're gunning it from a dead stop? Otherwise, a stationary object just sits there... because it's stationary.

The aircraft is stationary with engine running and brakes on and tail resting on ground. I would then throttle up to full power.

This was also an issue when doing maintenance. For engine ground running at full power, the tail of the aircraft was weighted down to stop the aircraft tipping over the front wheels.
You can see the "saddle bag" and weights hung over the fuselage in front of the tailplane in this still from a video of a WWII Spitfire ground run:
(YouTube)

Where exactly are the weights? It looks like the plane is tied down to the ground in those pictures.

@AndrewMedico, eh? The ground isn't heavy enough?

@Wossname clearly, you're underestimating the power of an angry Merlin engine being restrained for emergency surgery.

@AndrewMedico the weights are the two lumps of lead hanging either side of the fuselage. Lead is pretty dense stuff! The plane certainly isn't "tied down to the ground" - I've seen this done "live" several times.

Note the full elevator deflection. The pilot has the stick buried in his gut to make sure that the horizontal stab/elevator are generating significant down-force.

Depends on the elevator input
As long as the stick is held back, as it should be for any kind of runup, then the elevator would be applying plenty of downward force.
Now, without the elevator input to cancel it, the pitchover torque might nose over an airplane, and it might not take one of those big WWII engines to do it.
There is a force directed aft at the interface where the (brake-locked) wheels meet the ground that is equal to the force forward along the propellor  shaft. This force is multiplied by the distance between the extension of the prop shaft and the ground between the wheels to create the torque.
It's the same torque, but with opposite magnitude, as that which produces the hotrod wheelie maneuver. (And the torque is there with or without the wheelie, which is why FWD cars don't make good drag racers.)
It will be worse for an airplane with long gear legs that sits well off the ground.
If the static thrust or the elevator moment is high enough, it could happen. I've always wondered what would happen if I didn't hold the stick back on runup.
Update:
Addressing comments ... well, it might take a little unfavorable wind too, certainly airplanes have nosed over, see below, on (perhaps downwind) runups, but note that on the takeoff roll in a tailwheel airplane, the tail comes up quite early on, and that's with neutral stick inputs, so one can only imagine that it could happen pretty darn fast with any reckless amount of forward stick.
Apparently, even nose wheel airplanes can do it, I believe this is a Cessna on the runup pad at KSQL:

Could you estimate (or measure?) the airspeed at the tail when stopped but with the throttle at taxiing speed and at full throttle ?

Is really the jet produced by the propellor so strong for this to be true?

It depends on what position the elevator is in! As long as there's enough back pressure, there's not going to be an issue. If you were to push FORWARD, then the plane would go over onto its nose. I've got some experience with this in X-Plane, and it's definitely possible. If I was involved in a similar engine run-up, I'd want some sort of tie-down on the tail if the plane was to remain stationary, to avoid the possibility of the pilot bumping the stick and shoving the plane over.

Just because it behaves like that in X-Plane does not mean it did that in reality. Its not like X-Planes ground handling is the most accurate simulation.

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How does a jet fighter landing flare compare to a civilian plane?
How does a jet fighter flare compare to a civilian plane?

How low before flaring?
Do pilot keep track of pitch or AoA to know how much should be flared?
Is flare at last moment like in airliners, so after crossing runway threshold?
Will touchdown happen during the flare, or must the pilot bring the aircraft out of the flare to land more flat?

It's just 1 question. The bullet-points are just there to illustrate things in the answer I would like clarification of (which are all flare related).

It all depends on the aircraft for specific handling qualities, but for the most part they are flown just like any other large turbojet aircraft.
Some things to keep in mind about their specific handling:  Jet fighters with the thin, low aspect ratio wings are really power dependent and glide about as well as a brick.  Therefore they will be flown with power all the way to the runway until the roundout commences.
Like other large jets, fighters are flown at optimal angles of attack, the speed at which to achieve this is dependent upon the aircraft's landing weight.  This can vary between 110-160 or so knots depending on the aircraft in question.  The optimal angle of attack is available to the pilot in the form of an optical AoA indexer and either an AoA indicator or a graphic reference in the pilot's head up display (the 'E' Bracket).
Pilots will line up for final at Vref + 10 kts or so, align with the optical glideslope eg PAPI, etc and establish the optimal speed for the approach and power settings.  An ideal sight picture from the jet will be to line up on runway centerline, have the Flight Path Marker in the HUD aligned with the PAPI or numbers and the E-Bracket centered on the flight path marker.  Power corrections should be made to hold this sight picture as needed; in the case of a fighter, gently walking the throttles should be all that is required.  Roundout will be commenced considerably higher than one would in a light GA aircraft so that the jet arrives at the correct roundout altitude of 1-2 feet and correct attitude for the approach, once the roundout commences, power will be pulled to idle, allowing the jet to gently settle on the runway.  Flaring is not done quite like a commercial jet due to the difference in handling characteristics between the two due to the high wing loading, stores drag and a few other features found in fighters.
Most terrestrial based fighters are flared; naval fighters are often not and are flow to impact with the runway so the pilot can maintain proficiency in the techniques required to do arrested landings on aircraft carriers or log practice approaches with FCLP or Field Carrier Landing Practice.  Even terrestrial air forces who operate naval fighters in their air force - one I can think of right now is the Canadian Air Force - teach no flare landings to their pilots; not surprising as the CAF CF-18 syllabus was copied and adapted from the USMC F-18 training syllabus from MCAS Cherry Point.

I had the opportunity to fly an F/A-18 simulator about 15 years ago while I was in San Diego. It was an unbelievable experience that I will always remember. So, the instructor laughed at me because I flared it like it was a C-172, barely felt the wheels touch, which was very very wrong. When I started landing on the carrier I understood why. You literally slam the machine onto the deck. It also ensures that the weight of wheels switches are properly activated which is required to do a bunch of things with the airplane that need to happen.

I had the same experience going through egress school in a F4 around 1980. I kept trying to approach flat with the nose on the horizon and couldn't break the fall - crashing into the ground.

Viggen was usually landed in that way too - 5 m/s sink rate until rear wheel down. The rear wheels was in a bicycle (?) arrangement with the wheel going sideways towards middle of aircraft.

One difference is most military fighters approach with a very high angle of attack (except carriers where the aircraft is on the verge of stall) - note how far off the ground the nose wheel is on the fighter compared to the airliner. Civilian or transport aircraft approach nearly flat and touchdown with the nose wheel modestly off the pavement and much less incline.
I don't know about military transport, but fighters depend almost exclusively on AOA for energy management. I would guess the AOD (angle on the deck) is twice or three times as much for the fighter.
The other oddity (at least in the F4) is you need to land with about 60-70% power. That was hard to get use to. I kept wanting to chop the power over the fence - wrong thing to do. You need to fly it all the way to the ground.

Aren't you supposed to give a link to the photo source ? I would consider that as a minimum option, not even considering the licencing issue...

Approaches, carrier or otherwise, are not flown "on the verge of stall"...

We fly our approaches with the AoA indicator.  The flare is more sight picture, as the way many maneuvers are taught, and feel, so we aren't really holding off for a specific touchdown speed, but it often ranges in the 140 KIAS region depending on the specific jet and weight.  The mantra in your head as you are landing is "aimpoint, airspeed, aimpoint, airspeed" and the flare does not begin until you are over the runway surface.  We hold the flare until the mains touch the tarmac then let the nose down (except for touch-and-goes, where, unlike civilian training, we do not let the nosewheel down), the attitude is not flattened out before landing, flare until you are on the ground.  The mains are close together, so directional control is crucial on any fighter style aicraft.  I have a T-38 main landing gear tire in my garage that the mechanics gave to me after it blew out on one of my landings (nothing wrong with the landing, the tire had finally given up the ghost after having the crap kicked out of it by the hundredth student).  Yes, you can flare too much and scrape the feathers, so maintaining the correct sight picture is necessary.  Hope this catches all of your questions.

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Why not extend the downwind when first learning to land?
I'm supposed to turn base when I see my touchdown point is 45° off my shoulder. But I'm a newbie and final has a lot for me to handle all at once. I'm sure as I improve it will become easier, but what would be the harm if I extended the downwind by a short distance to give myself a little more time to think and adjust during final?
(I will ask my CFI this question also, but I thought I'm probably not the only person wondering it.)

Funny story: The same day as I asked this question, we had a radio problem that caused us not to realize we weren't hearing traffic, (we had been hearing it until we entered the pattern) and saw another plane unexpectedly on final while we were on base. My CFI had me do a 360º from base. It was a hairy landing, with gusty crosswinds and being aware of wingtip vortices, too.

All the planes I train in are Cessna 172s from the 1970s. The one I usually try to reserve is a 172R.

I see a lot of these sort of questions from students - why this, why that etc. And I remember getting frustrated during my training too. The thing is, before you know it you'll be past this point - you'll feel like you've got way more time, and you won't have to then relearn the pattern. Reality is, you're learning as you mean to go on - which is exactly how it should be done, and once it clicks you're sorted

... final has a lot for me to handle all at once

You don't need to extend your downwind, you need to prepare to land earlier in the pattern.
To be honest, flying a longer downwind is not a great idea.  If a single engine plane loses its engine, you will not make it back to the runway.
Circular or box patterns are there to keep you within gliding distance as you prepare to land.  Never$^1$ leave this safe glide zone with a single engine.
Now, landing is done in stages.  The better you do on the previous stages, the easier final approach will be.  Your goal in training is to roll out onto final at the correct glide angle at the correct airspeed.
So, prepare to land on the downwind.  For a Cessna 172, as follows:

Enter downwind at 100 knots, proper distance from runway (halfway up.
the spar, whatever

Passing the end of the runway,  reduce throttle, hold altitude, slow to
70 knots.  This is the first step experience will help you.  If landing
into the wind, slow to 70 a little earlier. Flaps 10.

Turn base, trim to 65 knots, radio call base, judge high or low.
Notice you have taken out "fast or slow". Energy is Velocity
squared.  Be consistent with your airspeed here.

If anything, err on the high side, that's what flaps are for, but
don't be too fast.

While on base,  make altitude adjustments with flaps and throttle as
needed, and hold track against the wind.

Roll out to final, and what is there to do?  Radio call final and drive
on down to your roundout over the runway, then land.

Practice and repetition of key steps will make it easier.  If you don't have it together on final, going around is a good option.
$^1$ Air traffic control or situations such as avoiding traffic are exceptions

"Never leave this safe glide zone with a single engine."  Did you mean to say never choose to leave this safe glide zone?  ATC frequently forces you to extend your downwind to accomodate traffic.

@KirkWoll  edited in footnote, thanks

The key to good landings is a consistent pattern—doing the same things in the same places every time.
If you fly the downwind and base correctly, with the correct speed, power and flaps, then you will roll out on final on the proper glide slope. If this isn’t happening, the correct answer is not to buy yourself more time on final to make major adjustments but rather to fix the problems you’re creating for yourself earlier in the pattern.

Once your skill level gets high enough, having a precise pattern/circuit is much less critical.  When I was bush flying I set up landings from 300 ft agl arriving at the lake under a cloud deck at cruise, circling around to get slowed down and put it on.  As long as I'm roughly on speed and slope at a hundred feet, how I got there doesn't much matter.

@JohnK Of course. But this was what I needed to hear when I was a newbie with the same problem.

Same same lol..

I don’t recommend doing so on a regular basis, but an extended downwind and a long final at a low-traffic non-towered airport can be beneficial for a student to learn configuration specific power settings,  correct glideslope and airspeed management techniques on final, etc.  this should not become the default technique for flying the pattern in, but if can be beneficial to fly it like that on rare occasions, especially when transitioning to a different airplane type for learning it’s behavior and quirks in the traffic pattern.
Outside of that, for a light airplane, it’s better to keep the pattern tight: downwind about 1/2-3/4 mile offset from the runway, then turn base with the threshold approx 45° off your left shoulder, fly base at 1.4 Vso, turn into final on airspeed, on centerline, on glidepath and stabilized as soon as possible, etc.  This does several things.

You learn how to fly a tight pattern, helpful for airports in confined space or with noise abatement restrictions.

You keep the flow of traffic in the pattern as expedient as possible without compromising safety of flight and flying unstabilized approaches.

You do not interfere with other pilots in the pattern.

This develops good pattern habits which will translate to better landings and can be applied to multiple airports.

If you’re having struggles with landings, you’re not alone. Most people working on a private pilot’s license struggle with this to a certain degree.  Landings are the most difficult thing you will be required to master as a pilot because there are so many variables involved and multiple skills and hand eye coordination must be performed simultaneously.  Keep working on it. You’ll get the hang of it!

Robert DiGiovanni's answer also mentioned staying within gliding range of the runway, for single-engine planes.  That sounds to me worth mentioning in the bullet list as a reason to normally do that.

Ah yes the old debate about using power on final or gliding on final. On the one hand it does teach students how to perform power off 180s in the event they need them, on the other hand it tends to make them deficient on short field landing techniques, and is virtually impossible when they start doing instrument training as well as flying larger, more powerful Aircraft.  I’m in the use power on final camp.

Pretty sure Robert meant as a safety back-up against the risk of losing your one engine, not that you should normally glide in.  If you have the option of spending your time in the air within dead-stick range of a runway or not, in range is somewhat safer, right?

@PeterCordes If you fly with power on final, then, if you lose power on final, you will probably end up short of the runway unless you're already on very short final when power is lost. If you stay within glide range, then you would always need to glide in (or else do a slip or something) on the final in order to prevent landing long.

@reirab: Right, that makes sense.  Perhaps Robert's point was that you're in glide range for more of the time?  Like most of the time you're in the pattern, not while you're actually on final, could that be plausible?  And/or at least in range of gliding to a flat part of the airfield, which often has some flat area outside of the runway proper, which a light single-engine plane can make good use of, especially if the ground isn't super muddy/soft.  (I'm not a pilot, just trying to make sense of the two different points these answers make.)

@PeterCordes Even in a normal pattern, you're in glide range most of the time on the downwind leg. Once you get out towards the end of the downwind (when you're about to turn base,) then you're probably out of glide range on most light singles in a normal pattern. It used to be common for pilots to be trained to fly tighter patterns for that reason, but engine reliability improved over the decades. The FAA recommendation is 1/2 to 1 mile out from the runway. They don't recommend flying tight, power-off finals, as it gives you less time to get set up and stabilized.

@PeterCordes Of course, they also don't recommend extended downwinds like what the OP was asking about for all of the reason listed across the answers here. More time out of glide range is just one of many reasons why extended downwind isn't a good idea unless necessary for traffic.

@reirab: Yeah, ok, so it does still belong on the list of reasons, probably near the bottom. (But not described as always staying in glide range, just more of the time.) That's all I was trying to clarify.  Thanks for that and the additional details.

The short answer is because the plane behind you is NOT extending their downwind, and you will mess up the pattern when everyone else has to adjust to your deviation from established standards.  Stick with it, you will get it!

That just kicks the can down the road to "Why is this the established standard". And the answer is "because that is how far you can glide if need be".

@dotancohen, valid point if you are taking a “5Ys” approach to answering in the most complete way possible. But I see a trend towards sometimes over answering simple questions here, so often I strive for brevity in the interest of providing balance. (as prefaced by "the short answer is...")  Thanks for the feedback though.

If your CFI has you doing 45-degree base turns as a new pilot, then he is instructing you incorrectly and you should find a new instructor.
Final approach in any aircraft is supposed to be done at a 3-degree glide slope and if you are turning base when you are 45-degrees from the numbers, then there is no way you will be able to do that (unless you were to make a very steep descent onto base which is improper). Your instructor should be having you do 3-degree finals, which requires you to be out about 2000-2500 feet out and having an altitude of about 100-120 feet when you turn onto final. You are required by FAR 91.87D(3) to follow visual indicators such as VASIs when you land, which means a 3-degree glideslope at most runways, so your CFI should be doing 3-degree landings exclusively when you are a beginner. If he is not doing that, you should get a different instructor.
I recommend that you work out the math so you confidently and completely understand the geometry of a landing. Here are the key constraints to be aware of:

Airspeed: 1.3 x Vso (FAA recommendation) minimum
Do not descend faster than 500 feet per minute
Maintain a 3-degree glide slope on final

(Note that the VASI at your airport will probably be set to a 3-degree glide slope unless your airport has unusual terrain.)
Work through the math to figure out where you should be to meet these basic requirements. If you do that, you will find that your instructor is teaching you to do steep descents on final which he should not be doing until you are more advanced. As a beginner you should always be doing 3-degree glide slopes.
Experienced pilots in small aircraft often do steep finals to land fast, but you should not be learning that until you are a lot more experienced and have soloed. It is both potentially dangerous and improper for a beginner to be doing steep finals.
As for the other pilots in the thread saying that you should be rushing your finals so as not to delay the other pilots in the pattern, ignore them. You should be learning the RIGHT way to fly and the jerkoffs in the Extras behind you will just have to wait.

Comments are not for extended discussion; this conversation has been moved to chat.

All discussion notwithstanding, this reference still needs to be fixed as it does not exist -- "FAR 91.87D(3)"

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Is it unusual to train with multiple CFIs?
I've been struggling to schedule my flight lessons. My current instructor tends to be available later hours, and the planes I've been training in book up especially fast during that time of day. I sometimes have weeks between lessons because of the scheduling problems.
I realize I could just switch completely to another instructor, but I basically like working with the instructor I have. On the other hand, it occurs to me I might stand to gain from getting some diversity of instruction style.
In some traditions, teacher loyalty is considered important. I don't get the sense that it's super important in aviation (especially since so many CFIs are just building time to ATP). But how unusual would it be for a student to regularly bounce between two instructors to make scheduling easy? Would such a thing be frowned on?

Welcome to aviation.SE! Does this question help?

In a very highly structured flight school with rigorously standardized curriculum, which is usually only seen in Part 141 schools, it’s common to switch instructors depending on who is available when you want to fly, and that ability means being able to finish in less time, which offsets the higher rates per hour.
However, in a Part 61 school or club, which typically use independent instructors with no standardized curriculum or testing, every switch will waste time and money because you will end up repeating lessons to convince each new instructor that all the other ones taught you things correctly.
If you’re in the latter situation, have a sit-down conversation with your current instructor about your scheduling issues and, if they can’t resolve it to your satisfaction, find a new instructor (or school) that can—and then stick with them until you’re done so you only pay the switching cost once.

Oddly, glider clubs tend to be more like the big schools as you mentioned and run on a more standardized (albeit much simpler) training syllabus, and as  a result it's normal to fly with every instructor in the club, which seems to work fine.  Not so much in power land at a regular ad hoc sort of flight school where the syllabus tends to be created by the individual instructor.

@JohnK Is it perhaps that all the instructors at the glider club had the same instructor(s) themselves, due to it being such a tight-knit community?

It's mostly that the training is more centrally regimented and standardized in glider clubs, based on national guidelines, not hard to do since the training is far more basic. On the power instructor course, part of the journey is developing your personal lesson plan structure, which will vary from instructor to instructor.  Switching instructors means switching entire worldviews at a small school since each instructor is a unique training program in itself you could say.  The glider environment tends to be very short lessons given piecemeal, making an externally imposed structure a necessity.


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What is the stress tolerance of landing gear of commercial aircraft?
Context: The landing skills of commercial airline pilots vary widely -- ranging from the ability to routinely make feather-touch landings and taxi and almost imperceptible deceleration on the one hand, to habitually landing with a medium-heavy thud and decelerating abruptly, even under normal flying conditions. One guesses that the landing forces on various parts of the landing gear (and also other parts) would momentarily vary by a factor of three or four, or even greater, on some key parts.
Questions:
1) Please elaborate on the stress tolerances built into various parts of the landing gear of common commercial aircraft, such as the Airbus A320 and Boeing 737?
2) What kind of stress testing is done on crucial parts of the landing gear, such as tyres, wheels, axles and hydraulic parts?
3) What are the backups and fail-safe measures designed into such landing gear?

Related: How much force does a 747 land with? and How would we calculate the load factor on an aerial vehicle while landing due to sudden gust loads? and Is the main landing gear of a large airplane designed for sideways (crabbed) touchdowns?

Note that hard landings aren't necessarily unintentional. Landing firmly gives you more traction more quickly, which can be especially useful on short or wet runways. The imperceptible touchdown (and especially imperceptible deceleration) landings use up a lot more runway. It may be more comfortable for the passengers, but it can also be less safe in some circumstances. This question goes into more detail.

First off, the thud and deceleration may be more related to the conditions as well as the runway length than pilot skill, but lets discuss some of your questions.

1) Please elaborate on the stress tolerances built into various parts of the landing gear of common commercial aircraft, such as the Airbus
  A320 and Boeing 737?

This is a complex question to answer simply as airplane gear sees lost of loads and some compromises are made for simplicity of operation. For one the rubber used on the tires is quite thick since some of it does wear away on every landing since the gear is not spinning when the plane touches down (you can read more on why not here) The tradeoff here is weight (you need to haul the tires around with you) but they can be used for many cycles. The vertical load is generally handled by an Oleo Strut these allow for a margin of error but they (or the gear system) can be bottomed out and a hard enough landing can result in a bounce. The last serious load that gear sees is usually side loads when a crosswind landing is made. These are tough to mitigate and its generally the job of the pilot to reduce these loads they will manifest them selves as tire wear and mechanical wear. Perhaps the coolest solution to this was the B-52's full rotating gear.

(Source)
One of the more interesting non Oleo designs for GA gear is the rubber disks used on Mooneys. These are said to require less maintenance and are easier to replace but are far less forgiving than Oleo struts.

(source)
Some of the vertical force is absorbed by the tire its self when the plane touches down the tire does ballon out a bit. As such it is of vital importance that the tire pressure is within a small margin since low pressure can cause excessive ballooning which would lead to side wear depending on gear design.

2) What kind of stress testing is done on crucial parts of the landing
  gear, such as tyres, wheels, axles and hydraulic parts?

These parts are generally made by different manufactures and are designed to comply with the aircrafts specs as set fourth by the builder (Boeing etc). Like everything else on a plane these parts have a cycle life (TO/LD) and are replaced when necessary. Since they are high impact I would assume the life is on the shorter side generally speaking. Here is some info from Goodyear on how to care and maintain tires for aircrafts. Heres some good info on Oleo struts as well.

3) What are the backups and fail-safe measures designed into such
  landing gear?

This depends on the plane, bigger planes generally need to spread the load out over more axles/tires which gives them greater redundancy a plane like a 747 has 2 sets of twin axle main gear on each side of the plane so you could lose an axle and still be ok (relatively speaking)

(source)
compare this with a 737 and you will see that the main gear is less redundant as its a single axle only system. In this case losing one of two tires would be less catastrophic than losing all the gear.

(source)
Compare this all to a smaller GA plane where you generally have a single wheel situation

(source)
The one thing all these planes have in common is that the nose wheel is the most delicate part of the landing gear. Hard landings on the nose wheel can collapse the gear or cause porpoising both of which are bad. This is what happens if the gear does not rotate properly in other words that nose gear saw complete side loads. Planes have be successfully landed with collapsed nose gear or nose gear that wont come down. Its a sight but generally speaking its not the worst thing to happen.
Sensors: The only sensor I know of on the gear (and this is plane dependent) is the squat sensor which is a simple on off for being on the ground or not. The FDR will record 3-axis acceleration but that will be of the airframe which is behind the Oleo struts so the gear its self may see higher loads.

Another thing to mention would be required over weight landing and hard landing inspections.  The gear is designed for a certain weight and if for some reason this is exceeded the maintenance procedures must be followed.

Dave, Excellent answer. Could you please provide image attribution?

Is there any sensor or accelerometer readings maintained to do some sort of cumulative gear stress calculation? i.e. The number of landings is a first order approximation but an integral over a stress - time curve might be more meaningful? With today's MEMS sensors it might not be impossible to actually evaluate the cumulative stress imposed on gear?

@curious_cat, every airliner (and most smaller aircraft with good glass cockpit package) have a set of accelerometers in all three axis as part of the inertial reference system that is used for attitude indicator, inertial navigation and autopilot and it can record the peak acceleration during landing too.

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text:
What are the statistical probabilities of commercial aircraft accidents?
Is there any study showing how probabilities of various kinds of aviation (e.g. crash-landings, fatal crashes, mid-air collisions) accidents have increased or reduced over time?

@RalphJ - True about the wealth of information. Not sure if this information is on Stack Exchange, though. Should I delete this query?

What is your definition of an accident? See: What is the difference between aviation “accident” and “incident”?

@mins - Accident as defined by ICAO annex 13. [http://www.iprr.org/manuals/Annex13.html ]

@RalphJ We prefer not to just send people to Google. Even if the information is available, many people have difficulty finding the right links and understanding them. That's exactly how SE can help.

@mins - Well, I was actually thinking about statistical probabilities. While boarding a plane, what are the chances of that plane being in an accident?

I'm sure the information is right here: See this, for example.

Short answer
The International Air Transport Association (IATA) publishes each year a report about aircraft accidents in the world. The latest one indicates these average figures:

There are 3.3 billion passengers per year.
There are 100,000 flights per day.

Over the last four years the accident rate decreased from 94 to 73 accidents per year.
In 2014, among these 36,500,000 flights:

There has been a total of 73 accidents
12 accidents had fatalities (641 fatalities).

ICAO also publishes statistics which can be viewed online according to custom criteria.

Extracts from IATA 2015 report

(Source: IATA report)
Period 2010-2014

(Source: IATA report)
Type and fatalities:

(Source: IATA report)
Geographical distribution:

(Source: IATA report)
ICAO statistics
ICAO publishes statistics which can also be viewed using a graphical web front-end:

ICAO accident statistics application
Data ("open data" series) can be accessed using the public API, but registration is required.


text:
How is lift provided by a wing affected by propeller wake?
Does the lift of that section of the wing which is in the wake of the propeller increase (because the airspeed in that section is higher)?
If yes, is this fact used actively to improve the lift capability?
Or does it decrease significantly because of turbulence and an overall disturbed airflow on the wing surface?

or decreased because of the turbulence of the blades?

Any reason to limit the answers to turboprops?

@SkipMiller : Turbojets and turbofans will guide the air entirely through a nacelle shroud through a passage without the fluid coming in contact with a wing surface (ignore interference drag). This is outside the premise of my question. You may use a piston engine if you want! All I need is that there's a blanketed airflow on the airfoil.

@Raj I think he meant why not also consider internal combustion prop engines

@ratchet freak : as I mentioned earlier, piston prop IC engines too.

It both increases and decreases, depending on the local direction of the propellor (e.g. on the upgoing side, or the downgoing side) . See for example this PhD thesis: Propeller Wing Aerodynamic Interference.
This thesis shows the following image:

Simply said, the velocity of the propellor locally changes the angle of attack, and thus the lift generated by the wing.
It is interesting to know that it is possible to locally optimize the shape of the wing to take advantage of this effect. Again the thesis shows the result, and it looks like this:

If you look closely, you can see that the wing is changed at the propellor location to accomodate the adjusted flow.

An excellent answer! Answers my first question well. But is this different in velocity actively used anywhere? Like, are the airfoils in the wake different from the ones in the outboard section (say, they are optimised for a higher flow velocity)? Shouldn't they be?

I have to say that these vertical velocities resulting from the prop generally do not increase the total lift. It increases the lift on the prop-upward side, and decreases on the prop-downward side. This thesis optimized the wing such that the final lift distribution was closer to the elliptical lift distribution. (e.g. trying to cancel out the effects of the prop).

Further more, I think that the $\Delta V$ of a prop is kept as small as possible (to maintain a high propulsive efficiency). In the thesis (page 19) I found a picture that (I think) states velocity increases of 6%. I don't think this will have a very large influence on the performance in the low mach range that props typically serve, but I'm not sure.

Can't open the attached  "PhD thesis on propellor interaction". Can you plz provide another link.  Thanks.

I don't have a link, I've added the full title so you can search for it yourself

According to this NASA Document the lift after the propeller is higher and it is used on purpose to create more lift.
Page-5

Wing-mounted propulsion systems have significant effects on the wing
  aerodynamic characteristics, and these effects are more pronounced
  when the highlift components are deployed. Various aerodynamic
  components contribute to the rise of these effects. Some of these
  effects are external to the wing performance and affect the
  measurement of the aerodynamic characteristics of the combined
  assembly. Examples of these effects are the propeller thrust, the
  location of the thrust line, tile size and location of the exhaust
  nozzle, and the thrust from the exhaust nozzle alone. Another group of
  effects are pure aerodynamic effects, such as the propeller
  slipstream and the flow past the nacelle and nacelle attachments.

Page-7

Results indicate that the lift coefficient of the powered wing could
  be increased by the propeller slipstream when the rotational speed
  (disk loading) was increased and high-lift devices were incorporated.

It is also stated, that the exhaust of the turboprop can also create lift.
For example:
An exhaust of a free-turbine "PT6A-67" turboprop of a converted Conair Firecat doesn't generate any lift.

But an exhaust of an Direct-drive "Rolls-Royce RB.53 Dart" turboprop YS-11 which is nearly on the wing, can generate additional lift and acceleration.

There is a lot of work going on at NASA right now to take advantage of the additional lift created by propellers on the wing, which allows the wing to be thinner for the same amount of lift. Look for a series of papers coming in June of 2015 on distributed electric propulsion where this is being exploited as a way to achieve additional efficiency.

A thinner wing doesn't by itself produce more lift, more camber does. Thin, unswept wings have a smaller useful AoA region and are heavier than a comparable, thicker wing (or need bracing). Thin wings (with sweep) are most useful for supersonic flight.

Perhaps if thinner wings can be made lighter you have an advantage?


text:
What is the difference between Fly-By and Short-Turn?
By all definitions that my team and I have found, fly-by and short-turn (with respect to military unmanned air vehicle waypoint based flight) mean the same thing.  Briefly, this allows the pilot to fly within a capture distance of the waypoint but is not required to fly directly over or through it.  In other words, the pilot can start turning before the waypoint is reached.
Current Definition:
A short turn will be performed at the turn location.  A short turn indicates that the vehicle may begin its turn prior to reaching the waypoint and fly-by it instead of through or over it.
Is there any nuanced difference between these two states so that I might further clarify this in my definition?

Is this related to RNAV? RNAV defines two waypoint types: Fly-Over and Fly-By. I've never heard the term short-turn. In fact, googling "short turn" waypoint gives this question as first result.

@DeltaLima Perhaps, but this relates to military aviation, especially with respect to unmanned aerial vehicles.

Please edit the question to define context. This way it's rather ambiguous.

What do you mean by the current definition? Is that your definition, or is that a definition you have from some reference.

@DeltaLima that is the definition that I am currently using for these terms.

I would drop the term short turn and only use Fly-By and Fly-Over in combination with fix or waypoint. A fly-by waypoint means that the pilot (or autoflight system) will anticipate the turn and therefore starts the turn before reaching the waypoint. When using a fly-over waypoint, the turn will not start before the aircraft has flown directly over the waypoint.

The context of your question is a bit unclear, but assuming that you're asking about navigation waypoints in the US then you're probably referring to these terms as defined in the FAA's Pilot/Controller Glossary:

FLY-BY WAYPOINT- A fly-by waypoint requires the use of turn
  anticipation to avoid overshoot of the next flight segment.
FLY-OVER WAYPOINT- A fly-over waypoint precludes any turn until the
  waypoint is overflown and is followed by an intercept maneuver of the
  next flight segment.

As per the comments, I have never heard of a "short turn" waypoint and it isn't in the P/CG.

Maybe add something about the role of lead-in radials on DME arcs?

@rbp I could but it doesn't seem directly relevant here: the question as I understand it is about the terminology of waypoints rather than the concept or purpose of leading a turn in general

Check out the LOMMA waypoint. It's a flyby because of the lead-in radial. http://www.terps.com/ifrr/may96.pdf


text:
What are the states that the landing gear can be in?
What are the valid set of enumerated states for landing gear?
My current working set is:

DOWN
DOWN & LOCKED
STOWED
STOWED & LOCKED
INOPERATIVE
CYCLING_UP
CYCLING_DOWN

Also, are there any definitions in which UP is distinct from STOWED?  (I can't find any.)

What is the purpose?

Primarily seeking to clarify my understanding of all valid states.

Are you asking as a pilot or a programmer? I'm asking because it sounds kinda like you're trying to create a model of a plane

Programmer.  I am developing a general model of an aircraft and need to know the common, acceptable states.  When I compose these states, I can get the instrument states (as documented in Jan's answer below), but I need the discrete states for my model.

if you allow the bay doors to malfunction then you can have up/locked and not stowed

@ratchetfreak So, STOWED implies the bay doors are closed and UP merely reflects the position of the gear?  As a result, I suppose it is possible to have STOWED & NOT LOCKED.  (That said, I am modeling bay doors as a separate entity.)

that's my interpretation of stowed at least

What exactly does INOPERATIVE mean? It seems like the landing gear could become inoperative at any point, and it's important to distinguish, say, landing gear that's down and locked but can't be moved from landing gear that's up and locked and stowed but can't be moved.

That's a good point, as well.  I may rethink INOPERATIVE.  That body of problems may well be subject to another status monitoring system.

@Chris: Ok, I have extended the question with some suggestions about simulating. Basically you need a value you will transition rather than a set of states.

@JanHudec Thanks for the additional detail.  I am building a data model (rather than a graphical one), but this is excellent information.  Awarding as selected answer.

Any instruments always show only three states:

Down and locked (green)
Up/stowed (extinguished)
Anywhere in between (red)

If you want to simulate it, remember that while the pilot can only command one of the two end states, the transition is a continuous process and you need to represent exact position of the gear in each frame of the simulation. So you need a value that will smoothly transition between 0 (stowed) and 1 (down and locked). Any value in between will indicate red on the instrument, but will correspond to different state of the gear animation.
The exact steps during gear extension (and in reverse for raising) differ by aircraft, but generally the animation will have three parts that you will map to suitable ranges of the gear state value:

opening the gear door,
lowering the gear and
closing the main gear door.

The last step actually complicates issues a bit, because the gear is already down and locked when the main gear door close and in case you do gravity extension the main gear door remain open.
So the suggested mapping is actually something like:

opening the gear door 0.0 to 0.3
lowering the gear 0.4 to 1.0
closing the main gear door 1.1 to 1.4

When you extend the gear normally, the value will transition up to 1.4 (and show green past 1.0), when you extend the gear via alternate mechanism (gravity drop), it will only transition to 1.0. If you want to simulate alternate extension, of course.
Tweak the values to make the animation look appropriate. The relative speed differs by aircraft. Some aircraft also don't have gear door (e.g. 737 only has one panel that is attached to the strut on main wheels; it still does have nose gear door, so the process will be slightly different for each leg).
Which leads me to note, that if you want to simulate failures, you definitely need separate value for each leg, because some failures, like jammed actuator, only affect one gear. Also in reality the legs take slightly different time to extend, so you can add some variability to the transition time if you wish.

Just a small note that bugs me every time: Red is not 'in between'. Red means 'gear position is not in agreement with lever position'

In addition, most gears do not move at a constant speed between up and down, but move faster when gravity supports the motion. Also, to minimize the demands on the hydraulic system, the left and right main gear on many planes do not move in sync, but with a notable time difference.

@PeterKämpf Another reason for the difference in speed is that hydraulic cylinders retract faster than they extend (with a fixed volume of hydraulic fluid) because the piston takes up room on the retract side and won't need as much fluid to move it.

@Radu094 The meaning of red varies with the aircraft.  In my airplane, for the main gear it simply means that the gear doors are not closed and has nothing to do with the gear itself.  For the nose gear, since it doesn't have a gear door that closes,  it means that it is in transit.

@Lnafziger: If you ever need to do gravity extend, it should indicate green when the gear is down and locked even without the door closed, no? Or would you get both red and green?

@Lnafziger, hmm.. I see. Always thought it's a design requirement. I mean, what happens if you lower the landing gear handle, but the doors stay shut (for some reason)? Do you not get a red light?

@JanHudec When we gravity extend (our second alternate extension method) we will have three green and two red lights. The nose gear red light will be the only one that goes out, since it doesn't have a door that is supposed to close.

@Radu094 In our case, We would get no red or green lights for the mains, but a flashing red gear handle as a warning (that warning is a disagreement light) as well as the gear warning horn under the appropriate conditions (full flaps selected, etc.).

To give an idea of the states that are possible along with the indications (and generally the complexity of the system) here are some tables from a 767-300.

Excellent, information.  This is certainly a great insight as to how this has been implemented and is being used in the industry.


text:
Why do many fighter jets have double vertical stabilizers?
Many fighter jets have double vertical stabilizers, e.g. F-14, F-15, F/A-18, F-22:

On the contrary, most general aviation planes and commercial airliners do not. While there are certainly exceptions in both categories, I think it is fair to say that this design is, comparatively, far more common in fighter jets.
What is the advantage of double vertical stabilizers? Why is it common in fighter jets?

Redundancy in case of combat damage.

As these planes push the edges of what controllable flight is, they need control surfaces that most planes don't. Many of these planes couldn't even fly without the complicated computer systems they run on, and the vertical "stabilizers" do way more than just control the yaw like they do on commercial/GA aircraft.

Since you've posted an F/A-18, here you can get an idea what else this vertical stabilizers do: http://aviation.stackexchange.com/questions/19214/f-a-18-rudder-position-during-takeoff

Advantages of twin vertical fins was thoroughly covered here: http://aviation.stackexchange.com/questions/24020/what-are-the-advantages-of-single-vertical-stabilizer-over-a-twin-set/

I'd also like to point out that this depends a lot on "design culture", i.e. all the examples you posted are from the U.S.A. In contrast, European fighter-jet design generally has a single vertical stabilizer, but in contrast has long emphasized delta-wings and canards, which are largely absent in U.S. design.

Though the reasons for having twin vertical stabilizers vary from aircraft to aircraft and is mainly a design decision, there are some advantages in having twin vertical stabilizers.

Having twin rudders increases the redundancy, which is important for combat aircraft in case of battle damage.
Having twin vertical stabilizers helps in reduction in height, though this is mainly an issue with naval aircraft. As an example, both the S-3 Viking and A-5 Vigilante had to fold their vertical tails for fitting inside hangars.

By USN - www.navsource.org 1, Public Domain, https://commons.wikimedia.org/w/index.php?curid=3109863

In case of twin engined aircraft with engines spaced wide apart, having twin rudders helps in getting sufficient rudder authority in case of an engine failure.
Having twin vertical stabilizers reduces their size, resulting in reduced stress at stabilizer root and reduction in weight.
One advantage of having twin vertical stabilizers is that the location of vertical tails helps in having control authority at high alpha (where the fuselage/wing can blank a single vertical stabilizer).
In modern stealth aircraft, having a twin tail enables them to be canted, helping in reducing RCS. Except for tailless aircraft (like B-2), almost all other stealth aircraft have canted twin tails.
Some aircraft, like A-10 had twin tails that were designed to mask engine exhaust from ground to ensure better survivability.

In the end of the day, having a single or twin vertical tail is a design decision and is taken based on the requirements. For example, preliminary designs of the A-5 Vigilante had twin tails, while the final design had a single tail, while it was the other way in case of F-14 Tomcat.

Also, from what I understood, the rudders serve also as "ailerons" and/or "speed brakes" when put in the opposite direction w.r.t. each other. Is that right?

@yo' you mean if they both face inside or outside? The primary effect observed is that it gives them additional "elevator" authority -- that is, they help the airplane pitch up more.  The amount additional drag or "speedbrake" effect is negligible vs. the pitching moment it causes. To see what I mean, look at the elevator: it also causes drag when deflected, but it primarily causes a large pitching moment. In addition to the many good reasons already listed, twin v-tails can help reduce overall mass by allowing the elevator to be smaller while the airplane can achieve the same pitching moment.

There are a few reasons many fighter/bomber designs have 2 vertical stabilizers:

one large tail produced more shear force at the root, 2 smaller tails mean less force at the vertical stabilizer root. This allows for a lighter structure
redundancy if one tail is damaged. The US navy philosophy is 2 is better than one, this makes sense given that its aircraft spend much of the time over oceans where there's few alternative landing sites
twin tails are shorter than one big tail, and vertical height is a consideration on carrier aircraft
twin engines spaced apart make for a wide fuselage back which can create a blanking effect with a single tail in the middle, 2 tails mean that there's always one vertical stabilizer that is effective
Twin tails create less of a radar cross section, and allow the tails to be angled to deflect radar signals away from airborne radar sources

Also smaller size means less radar reflecting surfaces.

True @Sami, I'm not sure it's a major reason for most designs but certainly for some.

I guess I would characterize it as a nice bonus.

"The US navy philosophy is 2 is better than one" -- Too bad they didn't get what they wanted out of the JSF program... :/

TBH you do have some issues with asymmetric controls if you lose one of the two tails in a tilted pair.

@MSalters I think that for an aircraft that needs a vertical stabilizer, an assymetric one is better than none, isn't it? :) I think it's similar to losing one of your 2 engines (in case you've got 2 of them)...

@SnakeDoc - Navy needs two engines Modern engines have come a long way and are quite reliable. In the last 25 years, the F-15 has suffered 5 Class A mishaps; the F-16 has suffered zero. They both use the same F100-PW-229 engine. --- Compare the early F-16 vs current F-35. After flying for 8 years, the F-16 crashed 50 airframes (nickname: Lawn Dart). In 8 years, we've lost just one F-35 (on the ground; engine failure due to excessive maneuvers before wearing in the engines). The Navy has successfully flown single-engine a/c before; I think they'll be fine.

@HephaestusAetnaean Having 2 engines isn't only about random engine failure. Having a second engine greatly increases survivability in a lot of scenarios, especially for aircraft stationed in the middle of an ocean on a moving boat. And to compare to F-35 losses is quite disingenuous, given that plane is barely even kind-of serving sort-of limited use in one branch. The F-35 is supposed to replace many airframes, including the A-10, and we know many A-10's have had engine failure due to small arms fire an more, but have made it back safely thanks to a second engine.

@SnakeDoc - A second engine also increases MX and chance for failures, which can cripple both engines if mounted adjacently --- I think the F-16/35 loss comparison is quite favorable to the F-35: the F-16 lifetime loss rate is around 4 per 100,000 hours. The F-35 fleet stands at 2 per 100,000 hours (1 loss on the ground in 50,000+ hours, 0 crashes), which is surprisingly considering the F-35 is still in development, when you'd expect accidents to occur more often.

@SnakeDoc - I won't get into the A-10 debate today, but plenty of other a/c (both single and twin engined) have sustained small arms fire and survived. But against SAMs, the track record is spottier. The A-10 is a beaut, but people are taking the "flying tank" reputation a bit too literally, and it's making it difficult to retire her once her time has passed. --- btw, I forgot what the F-16's loss rate was after its first 50,000 hours, but it was something pretty uncomfortable.

I think chat may be a better place for this discussion gents.

@HephaestusAetnaean We can't dismiss the A-10's record, since the F-35 is supposed to replace it. In a CAS role, the F-35 is going to get torn up, just like the A-10's already do. Having a second engine will undeniably increase survivalbility in this specific role. I'm aware the F-16 is officially billed as a "Multirole" craft, but in reality it's an air superiority fighter, and comparing it with A-10's is not accurate. F-16's also aren't based on carriers... and F-16's were designed to be cheap disposable craft - very unlike the F-35. But yes, GdD is right, we should take this to chat.

Chat room. --- F-16 annual attrition rate: 2% in first 5 years. Low of 0.18% in 2005.

the reason for two vertical tails is so the plane can maneuver during high angle of attack situations.  With a single vertical tail, you cannot turn ( maneuver ) during a high angle of attack situation, making the plane a strafing target.

Your answer could be greatly improved by adding some references.  Welcome to Aviation.SE!

Utterly untrue.  Every airplane designed for aerobatics (including high AoA maneuvers) has only one fin, not two.


text:
Which precautions should I take when flying in very cold / hot weather?
What are the important precautionary measures to take when a flight (either GA or airliners) is operating in extremely cold (e.g. -10°C or below) or extremely hot (e.g. 40°C or above) weather conditions?

Related for cold conditions.

Pre-heat the engine
When the engine is cold, the metals may come in contact to each other, which greatly increases engine wear. The best way to pre-heat is to use a hangar.
Engine oil
Oil viscosity is closely related to temperature. If the temperature is too low, oil becomes too thick. Thick oil cannot flow around the engine compartments freely. Thin oil, on the other hand, may have reduced lubricating effect. Thus, choosing the correct oil type is important. An oil with lower viscosity may be used in cold climate, and likewise oil with high viscosity in a hot climate.
Cabin heat
Cabin heat can be useful for warming the cabin, however a leakage may lead to carbon monoxide poisoning. Carbon monoxide detectors can be useful in providing an early warning.
De-ice
Ice built up can affect aerodynamic surfaces severely. In serious cases, they may cause the wings to lose enough lift and stall. Pitot tubes and engine sensors may also give false readings if their de-ice/anti-ice system is not activated.
Density altitude
The density of cold air is greater than normal. Engine damage may be induced by pushing full throttle, which causes the engine to produce more power than it is rated.
In hot air, the density is reduced. This reduces engine performance, resulting in longer takeoff and landing rolls. Aerodynamic performance is also reduced, resulting in a higher TAS (true airspeed) for the same IAS (indicated airspeed). These effects must be taken into consideration in determining whether runway length is sufficient.
If there is high terrain near an hot airport, it may be necessary to circle the airport to climb as the plane may not have sufficient climb performance.
Surface friction
In cold conditions, the taxiways and runways are often icy and slippery. Heavy braking should be avoided.

Is the longer take-off roll in hot weather because of decreased engine power or decreased lift from the wings? (I thought it was the latter.) Also, I thought that most braking on landing was from the wheel brakes, which shouldn't be affected much by temperature.

Great points but an answer for GA aircraft is not complete without talking about the human factors element in extreme conditions.

In addition to Kevin's excellent answer, there are a few additional factors I can think of.
In colder, denser air, lift is greater for the same aerofoil, angle of attack and airspeed and lower in warmer, less dense air.
In piston engined craft with normally aspirated carburettors, the temperature will affect the risk of carburettor icing which can be counter intuitive.  At very low temperatures, icing is much less likely than at warmer temperatures since very cold air contains little or no moisture.  In fact, in the UK, carburettor icing is a risk at some power settings throughout the year since there are few days cold enough to be carrying dry air and relative humidity is high for most of the time.
All of the craft I have seen which require carb heating to counter this have an inlet temperature gauge with a green section. The pilot applies heat to keep the gauge in the green which might mean removing heat as the temperature drops.  Heat is only used when needed since it lowers the density of the inlet charge to the engine which reduces available power.
The difference in all of these factors became very apparent to me when learning to fly an R22 helicopter.  On cold, winter days, it was a different machine, climbing like a rocket, greatly reduced throttle setting for a given attitude and speed and even more manoeuvrability than usual.  I remember once almost being in auto-rotation just to descend, she just didn't want to return to earth. Conversely, on hot summer days with my instructor, we sometimes had to do a running take off and would need quite a lot of power in the descent to keep the rate in check.
In short, most aircraft perform better when it's cold and dry.


text:
What are the mechanisms fitted in the SA321G Super Frelon rotor head?
Past the cyclic-collective pitch control, I understand (maybe incorrectly) that additional mechanisms are required to prevent or limit vibrations and loads on the mast and the blades.

Aerospatiale SA321G Super Frelon, photo by Burkhard Domke (source)
There are other rotor heads that are very simplified:

Bell AH-1 Super Cobra (source)
Can you explain what are the main mechanisms in the Super Frelon rotor head, and why the simple rotor head design is not used?

The simple rotor heads came first I believe. The more complex ones is what happens if you let engineers lose with the assignment "improve this".

@ratchetfreak, not really. Fully articulated rotors (which is what SA321 has) were there first. AH-1 (and UH-1) has the advantage of using a simpler semi-rigid design, which is only option for 2-bladed rotors. Modern rotors are often rigid, which is simpler, but  only became possible with newer materials with higher flexibility.

Holy cow! That looks like a network wiring cabinet run amok and expected to fly!!

@ratchetfreak The earliest prototype helicopters tried to use rigid heads, since they are simple to build, but suffered from a lack of understanding of these loads which could not be absorbed by the materials and manufacturing techniques of the time. The articulated head came first, as Jan Hudec rightly says, to reduce those loads on the blades and heads.  Indeed, rigid heads are the result of letting the engineers loose.

All rotor heads need to deal with multiple forces but two in particular have a big influence on the design and complexity of the rotor head.
As a blade moves forward (the advancing blade), it experiences an airflow equal to the speed of the blade plus the air speed.  As the blade moves backwards (the retreating blade), it experiences an airflow equal to the air speed minus the speed of the blade.
Therefore, the advancing blade generates more lift and will rise which causes the airflow to hit the blade with a more vertical component, reducing the angle of attack and the lift generated.  The opposite happens to the retreating blade.
This causes the advancing blade to flap up and the retreating blade to flap down.  The rotor head must allow this to happen and deal with the resulting forces.
The second effect arises from the Coriolis effect.  This results in a force applied to the hub by each blade as it speeds up and down called lead/lag.
There are three main types of rotor head.
Fully articulated.

From danubewings.com, copyright of FlightLearning.com
The blades are attached to the hub with hinges that allow the blade to flap up and down and lead/lag (or drag) hinges around the hub, allowing these forces to be absorbed without applying them to the blade or the hub. The blades may also rotate in a cuff through which the blade is attached to the hub. Each blade can flap and lead/lag.  Note that dampers may also be fitted to reduce vibrations and to better absorb the forces.
These are the most complex heads, an example of which is in your first image.
Semi-rigid.

Copyright www.copters.com
The blades are attached to the head with lead/lag hinges only.  Flapping is enabled by allowing the entire head to "teeter" on hub so that if one blades rises, the other must fall.  They cannot flap independently. Since the entire disc tilts together, semi-rigid heads can only have 2 blades.
The coning hinges allow both blades to move up and down together, without regard to where the teetering hinge is, as power is changed collectively.  So, the flapping (or teetering) hinge enables cyclic flapping whilst the coning hinge enables collective flapping.
Rigid.

Copyright helistart.com
The blades are rigidly attached to the hub and the flapping and lead/lag forces are absorbed entirely through flexing of the blades.  This is the simplest of all heads.
The choice of rotor head type is a complex one with intended design and performance being primary drivers but, as the head gets more complex, rigid being the simplest and fully articulated being the most complex, the maintenance cost goes up and generally, so does the weight.  Conversely, fully articulate heads produce the lightest loads on the main rotor shaft, it's thrust bearings and the gearbox coupling allowing weight savings to be made there. Rigid heads also require the strongest blades, and therefore weight, although modern composites are enabling the manufacture of blades with high flexibility but lower weight.
The head you have shown is particularly complex as it is six bladed and has an automatic folding system so that it can be stored and hangared in small spaces and be ground manoeuvrable on aircraft carriers with limited deck space. The tail folds at the same time as the blades. Most of the stuff you can see apart from the blades, their cuffs and their hinges is hydraulic piping, pumps and motors, electrical bonding strips, sensors and transducers. There is also an oil lubrication system for the bearings - you can see the oil reservoir on the top of the hub.  The use of elastomers in bearings has removed a lot of the need for lubrication.
If you stripped all of that it away, it wouldn't look (quite) so complex.  I.e. Not so barking mad!

Copyright helistart.com

Another excellent answer from our friendly, neighborhood rotor-wing pilot.

Right. The mere existence of Simon makes me work hard to think of more helicopter questions!


text:
How do you calculate the maximum distance to communicate with tower control?
I'm stuck on the following riddle found on the geocaching web site:
A helicopter pilot is flying at an altitude of 1981 meters. He is heading inbound an airfield 335 meters high. There are no particular obstacles (such as mountains, ...) between the helicopter and the airfield. What maximal distance (in nautical miles) can the pilot expect in order to be able to communicate with the airfield control tower?
Unfortunately I don't know anything about air control rules and multiple Google searches came to no avail.
Please could you give me a clue (not the answer as it would spoil the fun) how to answer this riddle?
Edit
For those who are interested here is the link to the geo-cache

I would say it's about the curvature of the earth and assumes the signal is blocked by the horizon

google for altitude / height / elevation and then VHF line of sight range calculations ...

I guess the "correct" answer is the theoretical maximum considering the curvature of the earth, but I doubt in reality the radio signal can be transmitted that far with adequate quality.

I wouldn't automatically assume that radio power is more limiting than curvature-of-the-earth.  It certainly could be, but plenty of aviation radios have adequate power to reach out a couple of hundred miles, and since we're talking pretty low altitudes here, those radios would have ample power for far greater ranges than what's being discussed in the answers.

Propagation for aviation bands was studied by ITU and factors quantified in REC P528 with several graphs and tables A zip file contains the related code and there is a graphical implementation on GitHub. As one can see it's not a just a matter of line of sight. From a height of 50m, radio horizon (including refraction effect) is at a mere 30 km, but VHF range may be several times this distance. Example.

For the first step try this line of sight calculator. Line of sight is an imaginary line that exists between two objects. Radio transmissions require a clear path between antennas known as radio line of sight. Formula:
Line of Sight = √ (2*height1) + √ (2*height2) in Miles

It would be more helpful to include some sort of explanation for how the calculator works, like the other current answers do, in case the link is broken or otherwise does not work for someone.

Here you go:Line of Sight Tutorial
Definition:
Line of Sight is an imaginary line that exists between two objects. Radio transmissions require a clear path between antennas known as radio Line of Sight.
Formula :
Line of Sight = √ (2height1) + √ (2height2) in Miles

Height in the formula above is in feet...

@Carnatic  Are those miles in your answer (and in the link) statute or nautical miles?

Radio transmissions in general do not require a clear line-of-sight between antennas. Most radio communications do not have line-of-sight between antennas. However, the signal will generally be significantly stronger within line-of-sight. When you actually have the Earth itself between the transmitters, that will usually cause enough attenuation that the communication won't work, which is why this formula works for aircraft radio transmissions. It's worth noting, though, that even this rule doesn't apply to all radio communications. Some curve with the ground or reflect off the ionosphere.

Just don't forget, that both are heights above ground. Substituting 1981 and 335 would not give realistic results.

1/ For radio LoS, Earth is 30% more flat that in reality (diffraction). 2/ If LoS is clear but 90% of 1st Fresnel ellipsoid is obstructed, a loss of 20 dB occurs. Its radius for 110 MHz and a distance of 50 km is 180 m, that's a lot when stations are on the ground (radio links towers in flat regions are tall for this reason). 3/ Overhorizon communication is still possible in VHF/UHF, with a loss of 0.5 dB/NM. 4/ There is no fixed range, it depends on the link budget (antennas, power used, losses, dry/wet air, receiver sensitivity, ...)

The formula for VHF range taught in flight schools1 is
$$ d = 1.25 * \sqrt{h} $$
where d is the range in nautical miles and h is the height in feet of the receiver above the transmitter.
So in your case it would be (spoiler: mouse-over to see hidden text):

 $$ 1.25 * \sqrt{(1981 - 335) * 3} \approx 87.84  $$

However, keep in mind that in some countries the range of Tower VHF radio tranmitters is limited so they only achieve a certain range. Uncontrolled airfields in Germany usually have a range of 30nm, which is achieved by limiting their power.
1: "Motorflug kompakt" - Winfried Kassera / 2012 - p. 296

taught in flight school - I think I need a partial refund. Anyway, Reminds me:  Flt. engineer asks capt. crusty what's our wing span?  Pilot: "I don't know". FE: "Don't you think you should know? What if you wanted to put the plane in the hanger?" Pilot: "Well, I don't know how wide the hangar is either."

@radarbob what's a 27 MHz CB radio ground station got to do with an aircraft mounted (as in: probably lower powered) 108-137 MHz radio? It's a good rule of thumb.

I'm afraid your formula is not correct. If you have an aircraft flying at 10000ft and a radio station on top of a mountain at 10000ft the range is 0?

@emilTthe formula is rather a rule of thumb, you cannot use it for extreme values like the one from your example.

@SentryRaven: The correct formula is 1.25*(sqrt(ht)+sqrt(hr)), because the height of the transmitter matters too. The higher the transmitter, the longer the range. The same goes for the reciever. That's because the range (ignoring transmitter power and obstacles) is limited by curvature of the earth. The higher you place any of the antennas, the longer it will take until the signal is blocked by the earth.

@SentryRaven I'm afraid neither your or Emil's formula validate on the geocaching site (which doesn't mean they're wrong, but looks like it's not the answer the geo-cache owner expected). Thanks anyway!

@Johann The question then is, which formula or algorithm did the geocaching site use? I see where Emils formula comes into play and I have found both formula equally on google for quick reference. The one I have in my answer is from my PPL exam preparation book, but I am looking forward to anybody who can shed a light on which one would be better. Maybe invite the guys from Physics.SE over?

The range is calculated by this formula:
$Range = 1.25 (√Ht + √Hr)$
'Range' - in nautical miles
Ht - height of transmitter in feet
Hr - height of receiver in feet

I tried this: 1.25*(80.618+33.152) which gives 142 nautical miles but unfortunately it doesn't seem to be the expected answer. Thanks anyway!

@Johann, you are substituting the wrong numbers. They are both heights (which in aviation means above ground), not altitudes.

You can't really apply this simple model in a situation where there's a substantially varying difference between height and altitude in the region of interest, because it assumes a spherical earth.  If one radio is atop an isolated mountain, that altitude is as good as height above the curvature of the earth.  On the other hand if both are over low terrain and there is a mountain in between, that may block a signal that would otherwise have line of sight across more typical curvature of the earth.

Line of sight calculation is based on earth radius and height above ground of transmitter (and receiver).  R = 21000000 (21 million feet) approximately.  Based on plane trigonometry, d squared = h * (2 * R + h).  This is distance to horizon for an observer at height h above surface of radius R.  If R >> h then a good approximation is:
d squared = 2 * R * h;
or
d = square root (2 * R) * square root (h)
and since R is essentially fixed, we can calculate it and are left with
d = 6480 * square root (h) = 6480 h^1/2
which gives d in feet.
Using another approximation 1nm (nautical mile) = 6076 feet
d = 1.07 h^1/2 (h in feet above surface, d in nautical miles)
or
d = 1.23 h^1/2 (h in feet above surface, d in statute miles)
So the well known formula: d = 1.25 h^1/2 is an approximation for the horizons range in statute miles when h is in feet.
The problem stated above is incomplete.  You are left with some assumptions when attempting its solution.
1) are we to assume adequate TX power and RX sensitivity to reach?
2) are we limited only be line of sight (i.e direct line between TX and RX that does not penetrate the earths surface?)
3)Should we assume the airfield height to be the height of the surface under the aircraft and at essentially all points between the two?
If all these assumptions can be made (maybe there are more?) then the height of the aircraft above the ground is 1646 m = 5400 ft and d = 81 nm.
Note that if you assume the airfield to be 335 m above ground level you will get a drastically different answer to the line of sight range.  With the aircraft at 1981 m (6500 ft) and the field at 335 m (1100 ft) then:
d1 (aircraft) = 89 ft and d2 (field) = 36 nm so d = d1 + d2 = 125nm.

I was taught similarly.  √ftALT×1.26 = Distance in NM. So 1981 m = 6500 ft, 335 m=1100ft, then difference =5400 ft. √5400*1.26=73.4*1.26=92.5 nm.

Convert first from meters to feet :
1981/3.28 = 603.96 =604 feet
335 / 3.28 = 102.13 feet

use formula  $Range=1.25(√Ht+√Hr)$ = 1.25 ( 24.55 + 10.10 )= 44.56 nm

You multiply meters by 3.28 to get feet, you don't divide.


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What is the relationship between thrust and engine RPM in a turbojet?
I understand that, to increase the thrust of a turbojet engine in flight, it is necessary to increase fuel mass flow rate. This throttling then increases the exit velocity of air and gives increased thrust (if this was wrong, please also explain how is the thrust practically changed in cruise).
How does the engine RPM vary in this process?
Is there an equation which binds together spool RPM to the thrust?

Can you specify which type of engine you are asking about?

Jet engine. Turbojet, specifically (I don't want to link the bypass fan speed to the thrust, instead something linking the HP or LP compressor speeds to the thrust ).

Do you need to know the relation in advance or are you allowed for experiments/lab-test?

First, lets see how thrust works. NASA explains it pretty good. They conclude that for turbo-fan engines "specific thrust [...] depends only on the velocity difference produced by the engine." If you think about it, velocity difference in a gas is dynamic pressure. So, how much thrust the engine will generate depends on how big of a dynamic pressure difference it can create in the air between its front and back. A turbo-fan engine creates this pressure difference in two ways. One is by using a fan to "push" air to the back. The other one is by heating up the air that passes through its burner and shooting it out of the nozzle (after passing it first from the turbine.)
Now, on to your question. We need to see if added RPM creates added thrust (or if more thrust needs more RPM.) The answer is that, yes it does. Providing the engine with more fuel will create faster escaping hot gasses from the burner. Those will create a bigger dynamic pressure difference between the burner and the nozzle and the turbine will turn faster. Power from the turbine is moved to the fan which will also turn faster thus pushing out more air.
Refer to here for a graphical description of the engine compartments and how pressure is formed between them.
How RPM and total thrust for a model turbo-fan engine are related is graphed on page four and seven of this document. There you can also find various equations that describe those graphs.
EDIT: A turbo-jet engine works pretty much the same but instead of using the power generated from the turbine to mainly move the fan, it uses it to move only the compressor. All its thrust is generated from the hot gasses coming out of the nozzle.

The relationship: non-linear. I know that at least for early jet engines (yes, I am that old.) 90% of the thrust was in the last 10% of RPM.
These engines also spooled up slowly. We were told to fly approaches at 70% RPM, because at that RPM, there was essentially no thrust. To go from idle to 100% took about 5 -6 seconds: most of that time getting to about 70%.
Moving the throttle forward fast meant watching the fuel flow go up, TIT increase and watch the RPM increase on time lapse photography.
Go-arounds had to be planned well in advance.

Welcome to Aviation.Se!

A turbojet works by compressing air, heating the compressed air while keeping pressure almost constant, and running this heated air through a turbine which will  expand it again. The work done while expanding air in the turbine is needed to run the compressor, and the heating in the middle is needed to add enough energy so the turbine will spin and the gas leaving the engine will still have more impulse/momentum (speed times mass) than the air which entered the compressor. At the end of this process, the air leaving the turbine will turn the remaining pressure into speed in the exhaust nozzle, so the gasses leave the engine at a higher speed, but the same pressure with which they entered the engine.
Adding more fuel to the combustion will increase the heat and speed of the gasses entering the turbine, so more work can be done here. This excess work will speed up the turbine, and with it the compressor, so more air gets pumped into the combustion process, until a new balance has been attained where the heat energy added in the combustion will just be enough to keep the engine spinning at its new speed. RPM and fuel flow go up and down together, but within strict limits and not linearly. Increasing fuel flow when the intake cannot provide more air, or increasing it too quickly to allow for spinup, will lead to a compressor surge, flow separates at the compressor blades and mass flow through the compressor collapses. Shortly after, the turbine will start to melt from the excessive gas temperature. To avoid improper throttle commands, modern jet engines are filtering commands through a computerized control system, so the pilot has fewer opportunities to screw up.
Every stage of the compressor adds a pressure increase, and faster rotation speed means more air throughput and an increased compression ratio. By spinning faster, the compressor can increase the pressure in the combustion chamber and increase mass flow, diluting the fuel-air mixture. Now the air entering the turbine flows faster but is cooler, because the same amount of fuel has to heat up a higher air mass per time. The turbine works like a compressor in reverse: After each turbine stage the air is expanded and cooler than when it entered this stage. The turbine will spin as fast as the energy given off in the expansion process allows it to turn the compressor, and the mass flow and entry temperature determine how much energy is available.
The rotation speed of the turbine at idle is roughly 50% of the speed at full thrust. The relation between thrust and RPM is rather complex, if you want to study it, I recommend simulation software like GasTurb.

This is quantifiable? Thrust as a function of RPM (and other terms)?

This is a good answer, but as it stands it's more of a response to the other answer.  Could you possibly expand on the comments you make on how RPM effects this?  Since that was the original question?

@Raj: I recommend to use GasTurb. I do not know of a simple equation.

Cool! Thanks. :)

@JayCarr: I hope I understood your comment correctly and added one more paragraph; please help me to see what is still missing!

Well, I know what I'm curious about is if engine RPM and thrust are strictly proportional or if there are a lot of mitigating factors.

@JayCarr: Both go up and down together, but within strict limits and not linearly. Increasing fuel flow when the intake cannot provide more air will lead to a compressor surge, and flow collapses. Shortly after, the turbine will start to melt from the excessive gas temperature.

I'd say chuck that into the answer and it would be good.  Though I'm not the judge on this one...

How does the engine RPM vary in this process?

It is safe to say that in most operating conditions the shaft-speed will increase with higher thrust settings. Here is why:
When we look at a very simplified version of the processes in a jet-engine, increasing fuel-flow will increase temperature in the combustor. This will push point 4 to a higher T.

At first the turbine will consume the same amount of energy. This means the »distance« between point 4 and 5 will stay the same. Finally, this means the »distance« between point 5 and point 8 is bigger. In a T-S-diagram the distance is a measure of energy stored in the air, so there is more energy in the air exiting the nozzle so the jet will be faster and this will increase the airspeed. BUT, another thing is happening as well, actually it is happening simultaneously but a bit slower: since the air is hotter at the turbine entry the turbine will also extract more energy from the hot air and will therefore speed up. Which leads to a higher speed in the compressor as well, which leads to your second question.

Is there an equation which binds together spool RPM to the thrust?

The short answer is: No
But, actually it is not that easy. There exits no single equation for the engine but a system of equations. The character of this system is non-linear an holds a large number of parameters (e.g. compressor- and turbine-efficiency curves) which are sometimes unknown and need to be measured during an experiment.
However, it is possible to perform experiments and reduce the number of parameters and build correlations. Which is done regularly in order to model engine performance or control the engine.
I am not entirely sure what kind of »equations« you were referring to in your question. Because in principle correlations are equations as well but one needs to fit those correlations based on experiments to determine the parameters which you cannot know in advance.
The above is a very simplified view on a jet-engine. Depending on the type on jet-engine there are a lot more settings which influence the engine-behaviour: for example the Variable-Guide-Vane-Actuation-Law, or the Bleed-Air-Schedule, or the Cooling-Sytem of the casing. They all contribute to the complexity which make a »simple equation« impossible.

thrust is directly proportional to the RPM and the Formula is Thrust~(RPM)^(3.5).
so if rpm increase thrust increases.

Of more importance to the pilot in monitoring engine performance is the Engine Pressure Ratio (EPR) gauge.EPR is a ratio of compressor inlet pressure against exhaust pressure which shows the efficiency of the propulsion process front to back.  EPR is measured in percentages, 100% being maximum thrust required to take off safely at a given aircraft weight, runway length and under prevailing weather conditions.
Of secondary concern to the pilot is the Turbine Gas Temperature (TGT) sometimes  termed Exhaust Gas Temperature (EGT) or Turbine Inlet Temperature (TIT). Higher life engines will exhibit higher TGT up to a maximum safe limit for turbine blade life.
Also of interest is the performance at a given fuel flow as measured on the fuel flow meter. RPM monitoring is of relatively insignificant performance provided max speeds are not exceeded during take off power. A high life almost worn out engine will not be able to produce its required take off EPR on a hot day, fully loaded aircraft on short runway without exceeding either TGT or RPM max limits. However, the same engine may produce sufficient EPR on a cold day, light loaded aircraft on a long runway. Pilots of 1970,s generation engines had to refer to charts when selecting "TAKE OFF EPR OF THE DAY".  Modern FADEC engine controls make all the calculations and adjustments automatically.


text:
What considerations should be taken into account for landing on gravel airstrips?
For the pilot of a Cessna or Piper 4 seater, what techniques and other considerations should be given before operating on a gravel/dirt airstrip?

Operating in general, or Landing? You title says one but your question asks another.

Standard soft-field landing and take-off techniques work on dirt, gravel or grass. The differences are more in ground operations, and even then they aren't really big differences: it comes down to being aware of what's around you and what condition the surface is in. That includes taxi and parking areas, not just the runway.
With gravel, the main risk is damage to the aircraft (especially the prop) or nearby people, vehicles or objects caused by gravel being thrown up by the prop. With dirt, you're more likely to have to worry about drag or even getting stuck, especially if the surface is wet.
Some specific considerations for gravel are:

If you're parked on gravel, be careful where the aircraft is pointed when you start up. You don't want to spray some unfortunate person, vehicle or building behind you. Re-position the aircraft before starting up if you have to, and you could also brush gravel away from the prop.
For the same reason, pick your runup spot carefully
Keep back pressure on the controls all the time while taxiing, to keep the prop as far away from the surface as possible. Don't feel you have to taxi in a straight line: taxi wherever the gravel is lightest and the surface is smoothest.
Open the throttle smoothly for takeoff or runups to avoid suddenly throwing up gravel

Dry, well-packed dirt usually isn't much of an issue (apart from visibility), but wet dirt - i.e. mud - is potentially tricky:

If conditions are very dry then you can throw up a lot of dust. That may not be as dangerous as spraying gravel around, but it's still unpleasant for anyone nearby and there is a safety risk if people can't see you or other aircraft/vehicles because of it.
If conditions are wet then there's a risk of getting stuck in mud. Once you're moving, try not to stop unless absolutely necessary. Consider doing a 'rolling runup' if you're confident you can do it safely (practice on tarmac first!).
Calculate your required takeoff roll carefully and know exactly where you have to abort. Mud will create a lot of drag, and you don't want to push your luck on what is most likely a short runway.
If you take off from wet dirt you'll leave ruts behind you. When the dirt dries out those will still be there, and they could be a hazard to other aircraft using the runway (some dirt becomes very hard when it dries). Try to find someone who can smooth them out after you depart.
If you do get mud on your aircraft, clean it off if at all possible before taking off. As well as decreasing performance, it could clog up pitot-static ports or otherwise affect airworthiness.

One thing I'd add when operating in a muddy dirt field is that the leading edges of your wings and flight control surfaces can get covered. This interferes with smooth air flow and can decrease performance significantly, so clean your airplane's wings and control surfaces if they get mucky.

One thing I'd add when starting on gravel surfaces, shutdown/start the engine while pointing into the wind. This will help prevent picking up rocks into the prop and damaging it.

Also some gravel is pseudo-pavement - it's coated or "chip sealed" to prevent hurled stones.

Remember to add that second notch of flaps (piper or Cessna) and fly the plane into ground effect before climbing out. 2. A soft field takeoff is not a short field takeoff.

Gravel and dirt are two totally different things. Gravel can damage or ruin a propeller. With gravel you normally try to take off as gradually as possible, rather than ramp it up to full throttle immediately. Also, you do not do the run up on the gravel if at all possible.
With dirt or grass you do what are called soft field take offs and landings. This should have been covered in your instruction. If you have a pilot's license and do not know how to properly take off on a grass strip then something went wrong.

I appreciate your concern Mr Durden but I personally have almost 90 hours flight time, the last 30 of which have been based at an airport with a grass strip. Soft field operations are not a mystery to me, but the first half of your answer is helpful.

Then why are you asking about dirt airstrips?

@TylerDurden, dirt is not the same as grass. That is presumably why OP is asking.

Also, your last sentence is unnecessary and combative for no reason.


text:
Why not use the engine of a model aircraft for other purpose?
I was reading the DLE-120 engine User Manual and saw this sentence:

This engine was designed for use in a model aircraft. Do not attempt to use it for any other purpose.

Why? Why not use the engine of a model aircraft for other purpose? What does mean "model aircraft"? Does this mean only RC aircraft or RC and UAVs?
Is there any technical or safety reasons why this engine could not be used in a different application, not involving a model aircraft?

FWIW, there is no regulatory teeth to this disclaimer in case you were pondering that issue.  If you bought it you now own it; use it to power your blow dryer, blender, or whatever you like.

But opinion or tribal knowledge based answers are always being dumped on, ergo, they go in comments.

Somewhere out there is a Homer-Simpson-a-like who can proudly say, "They have to say that because of me"

A possible technical reason might be that it needs the airflow for proper cooling.

@jamesqf Perfect comment!

You don't need to mount an engine in something other than a specific type of aircraft to give it proper airflow. And if that were the case the instructions would say that.

A lawyer wrote that. It means if you use it for any other purpose, they are not liable and you are.

I suggest you to add @jamesqf 's comment to your answer.

No. That makes no sense.


text:
Why was this helicopter denied entry to Class B airspace?
This question is somewhat related to this other one.
I listened to this exchange between a helicopter and Newark.
The helicopter wants to land at Newark. The controller tells the helicopter to remain clear of the Class B. I'm aware that the controllers must give clearance to operate in certain classes of airspace, and the helicopter wasn't granted clearance to do so.
Why was the helicopter denied (as far as can be deduced)?
What should the pilot have done differently, either to get clearance to land at Newark or to anticipate not being able to?

According to the post at LiveATC, Newark was closed, apparently due to smoke in the tower. With the airport closed, nobody could be granted clearance to take off or land.
Outside of declaring an emergency, there's nothing the pilot(s) could have done to get clearance at Newark other than just waiting.
In an emergency situation like smoke in the tower, there wouldn't be anything published beforehand for the pilots to look up, but otherwise, pilots look at NOTAMS during their flight planning for planned closures or outages. Even in good weather, proper flight planning would also involve an alternate airport, anticipating the possibility that the intended airport might be closed for an emergency like this.
Eight miles south of Newark is Linden Airport, a small uncontrolled field just outside the Class B surface area. If I had passengers in my ship who were trying to fly out of Newark, I would have radioed operations and asked them to send a limo to Linden to pick up my passengers.

[There is one part of the recording that says "[inaudible]". The pilot is announcing his position at the Statue of Liberty, a reporting point for copters landing EWR.]


text:
When do pilots deploy landing gear?
The landing gear should be down in order to qualify as a stabilized approach. Other than that, what are the guidelines on when to deploy landing gear on approach?

"Before contact with the ground" is generally considered to be optimal, and has noise abatement benefits (what's louder: the gear horn, or the sound of metal dragging on the runway?)

Just opened this site to ask pretty much the same question and yours was the first of the top questions :D

@voretaq7 In addition to the noise abatement concerns, I hear it also has significant operational cost and even customer satisfaction benefits.

I have never flown P1 in a retractable gear a/c, but the downwind checklist had it gear as the second item (brakes, undercarriage, mixture, pitch, carb. heat, fuel, flaps, gauges, hatch, harness) :-).

As late as possible to still meet your company's specific stabilized approach guidelines (mine were 500 ft VMC, 1000 ft IMC).  If you need the drag, then use them earlier.
In the EMB-145 in still wind or a headwind you could cross the FAF at 250 kts and clean and be on speed and configured by 1000' if you dropped the gear at the FAF (and opened the speed brakes and the first notch of flaps all at once).  Normally though, I'd drop the gear around 1000'.
Acronyms:

VMC = visual meteorological conditions
IMC = instrument meteorological conditions
FAF = final approach fix
EMB-145 is the type designator for the Embraer 145 regional jet

Could you define the acronyms?

@CGCampbell VMC = visual meteorological conditions, IMC = instrument meteorological conditions, FAF = final approach fix, EMB-145 is the type designator for the Embraer 145 regional jet.

In my airplane, dropping the gear at Vle at the FAF will give a nice fast stabilized approach

In light general aviation aircraft, you lower the gear when abeam the touchdown point on downwind, then double-check that the gear is down on final (and possibly on base, as well).

On light general aircraft, the gear is down and welded. The number of retracts in service is less than 5% of the fleet, and gear deployment techniques vary

That’s what I do as well. Gear, first notch of flaps, and reduce power together so you don’t climb or descend a bit when deploying them.  Then slow the airplane and  begin a 500'  per minute descent to landing.

It should normally be dropped about 500ft - 1000ft and below the aircraft's maximum speed for dropping the landing gear so you don't damage it.  If you need more drag to slow down quicker you can drop it a bit higher up.


text:
Can a missile be fired when a fighter jet is inverted?
Is there an attitude limit for firing a missile from a fighter jet? Can the pilots fire them when they're pulling Gs, or inverted?

The combat advantage of fighters (as opposed to heavier-payload bombers) is their maneuverability, therefore a missile delivery restriction such as g-limit (think Sidewinder heat-seeker in a dogfight) or attitude would unnecessarily restrict their efficacy.
As a Vietnam-era fighter and attack pilot I fired both unguided (aim and shoot rockets) and guided (target lock-on and shoot) missiles, and neither had associated attitude limits. Missiles that fall clear of the weapon station before rocket-motor ignition (such as the TV-guided Walleye or the optically-pilot-guided Bullpup) are obviously restricted from negative g to ensure safe separation from the airplane, i.e., negative g release from an under-wing weapon station would all but guarantee impact with the airplane.

Yeah -- stores separation is hard, just ask Pete Purvis about it...

I'd say that free-fall-to-ignition missiles are restricted from sub-1g, not just negative g (as trying to launch something like that in a 0g condition would be a repeat of Pete Purvis' bad experience...)

If inverted, just pull enough and you are in positive gs again.

Yes.  Specific weapons can be employed within specific limits and load factors of the flight envelope, some of which includes inverted flight for air to air missiles, as this F-22 test pilot demonstrates during the Raptor's flight test.

I didn't thought this is possible, now I see: it is, learned something :)

Yes, Modern air to air missiles can be fired while manoeuvring but it can reduce kill probability. So missiles can be fired when aircraft is inverted but within certain g-limits. Firing a missile while manoeuvring also depends on altitude of aircraft, type of missile and effectiveness of seeker.

Welcome to aviation.SE! This is good information, do you have any sources or references for it?


text:
What does the term "white-scarf Air Force" mean?
Ron Gordon's answer to question about A-10 cannon mentions "white-scarf Air Force". As civilian, subtlety was lost on me. What does it mean? From the context, looks like fighter pilots? Are the scarves of other colors, and what they are?
In particular:

"The white-scarf Air Force has an averse to building flying tanks and particularly doesn't like single-mission, CAS systems. They never will forget that the A-10 was forced on them."


text:
Is there a wake turbulence difference between B737s?
I've heard pilots talking about how the new 737s produce much more wake turbulence then the model historically. They described it as similar to the 757, based on anecdotal experience. Have there been any studies of this, or is there a quantifiable difference between 737 models?